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Show 244 DISINTEGRATION CnAP. V. they are formed. Through these several means, minute fragments of rocks of many kinds and mere particles in the soil will be continua11y exposed to chemical decmnposition ; and thus the amount of soil will tend to increase. .As worms line their burrows with their castings, and as the burrows penetrate to a depth of 5 or 6, or even more feet, some small amount of the humus-acids will be carried far down, and will there act on the underlying rocks and fragments of rock. Thus the thickness of the soil, if none be reInoved from the surface, will steadily though slowly tend to increase; but the accumulation will after a time delay the disintegration of the underlying rocks and of the more deeply seated particles. For the humus-acids which are generated chiefly in the upper layer of vegetable rnou1d, are extremely unstable compounds, and are liable to decomposition before they reach any considerable depth.* A. thick bed of overlying soil will also check the downward extension of great fluctuations of temperature, and in cold countries will check * This statement is taken from Mr. Julien, 'Proc. American Assoc. Science,' vol. xxviii., 18'l!l, p. 330. CnAP. V. AND DENUDATION. 245 the powerful action of frost. The free access of air will likewise be excluded. From the. c several causes disintegration would be almost arrested, if the overlying rnould were to increase much in thicknes , owing to none or little being removed from the surface.* In my own immediate neighbourhood we have a curious proof how effectually a few feet of clay checks some change which goes on in flints, lying freely exposed; for the large ones which have lain for some time on the surface of ploughed fields cannot be used for building; they will not cleave properly aw.l are said by the workmen to be rotten. t It is * 'l'he preservative power of a layer of moul<l and turf is often shown by the perfect state of the glacial s<.:ratche:; on rocks wheu firs.t uncovered. Mr. J. Gcikie maintaiu ·,in hill last very inter~ tmg work(' Prehistoric Europe,' 1881), th<tt the more perfect scratches arc probably duo to the last access of cold and increase of ice, during the long-continued, intermittent glacial period. . t Many geologists have felt much surprise at the complete d1s~ppearanc~ of flints over wide and nearly level areas, from whiCh the chalk bas been removed by subaerial dcnurlation. But the surface of every flint is coated by an opaque modified layer, which will just yield to a steel point, whilst the freshlyfractured, trans! ucent surface will not thus yield. 'l'he removal by atmospheric agencies of the outer modified surfaces of freely. exposed flints, though no doubt exccs:sively slow, together w1th tl:e modification travelling inwanls, will, as may be suspected, ultimately lead to their complete disintcoration notwithstanding that they appear to be so extremely d~rable., |